In connection with complication of tidal current in a electrical power system, control protection has been recently required to high-reliability and high-precision power system, and the necessity of enhancing the performance of a synchrophasor and a phasor measurement unit (PMU) has been increased.
A reference wave comparison system and a one-cycle integration system are mainly known for a synchrophasor measuring device used in the control protection for this type of power system. According to the reference wave comparison system, PMU has a reference wave of a system rated frequency and a GPS time therein, input data is compared with the reference wave and the difference between the input data and the reference wave is set as a synchrophasor. A problem of the reference wave system resides in that an error is very large because this system is a difference calculation system based on only one point (the difference calculation between the input data and the reference wave point at the same time) and thus no stable value is obtained although averaging processing or the like is executed (see Non-patent Document 1).
According to the one-cycle integration system, the self-end synchrophasor is calculated by integration calculation of one cycle. The calculating equation of the one-cycle integration system is as follows.
  V  =                    2            N        ⁢          {                                    ∑                          k              =              1                        N                    ⁢                                    V              k                        ·                          sin              ⁡                              (                                  k                  ⁢                                                                          ⁢                  θ                                )                                                    +                  j          ⁢                                    ∑                              k                =                1                            N                        ⁢                                          V                k                            ·                              cos                ⁡                                  (                                      k                    ⁢                                                                                  ⁢                    θ                                    )                                                                        }      V: phasor expression,Vk: sampling data,N: sampling number per periodθ: sampling angle,θ=tan−1 {Im(V)/Re(V)}This system has a problem that the calculation error of the above calculating equation is very large when the system frequency is deviated from the rated frequency (see Non-patent Document 2).
As described above, both the present two systems imitate an alternating-current waveform on the basis of a real-number instantaneous time. According to these methods, for example, a voltage instantaneous value waveform is discomposed by Fourier transform, a fundamental wave at the center thereof (synchrophasor) is extracted, and then the frequency of the fundamental wave is determined. Furthermore, it is also required to correct each alternating-current electric quantity on the basis of a frequency-gain characteristic curve. As described above, in the conventional systems described above, a delay time element such as Fourier transform, frequency-gain correction processing, averaging processing or the like is introduced, and thus no real-time measurement value is obtained. Accordingly, these systems have a drawback that it is impossible to measure a high-speed and high-precision synchrophasor and a phase angle difference between bus lines.    Non-patent Document 1: “IEEE Standard for Synchrophasors” C37. 118-2005)    Non-patent Document 2: “IEEJ Trans. PE, vol. 123, No. 12, pp 1471-1479, 2003”    Patent document 1: PCT/JP2007/052967 (Previous Application of this invention)